1. Field of the Invention
The present invention relates to an image forming apparatus and, particularly to an image forming apparatus that forms an image by an electrophotographic system.
2. Description of the Related Art
In an electrophotographic image forming apparatus (such as an MFP (Multi Function Peripheral) including a scanner function, a facsimile function, a copying function, a function as a printer, a data communication function, and a server function, a facsimile device, a copying machine, and a printer), it is necessary to keep a component used in an imaging unit in good condition in order to maintain good print quality for a long period of time. Therefore, a cleaning device is attached to a photosensitive body (image bearing body) that largely affects the image quality. The cleaning device includes a rubber blade that contacts with a surface of the photosensitive body to remove toner, which is not transferred, from the surface of the photosensitive body.
As to the cleaning device, it is well known that chatter or peeling of the cleaning blade occurs depending on an amount of toner or external additive, which is accumulated between the photosensitive body and the cleaning blade, and that performance of the cleaning blade largely depends on the chatter or peeling of the cleaning blade. In order to effectively operate the cleaning device to keep the surface of the photosensitive body in good condition, it is necessary to maintain the amount of toner or external additive accumulated between the photosensitive body and the cleaning blade at a constant value.
The amount of toner or external additive accumulated between the photosensitive body and the cleaning blade depends on an amount of residual toner or a charging state of the residual toner on the photosensitive body. In other words, in the state in which the image forming apparatus is actually used, the amount of toner or external additive always changes according to type (such as a kind of the image and a coverage rate) of the image printed by a user. For example, when the image having the low coverage rate is repeatedly printed, a small amount of toner is supplied to the cleaning device.
When the amount of toner supplied to the cleaning device is decreased, the performance of the cleaning device is tentatively degraded. Therefore, when the print is performed after the image having the low coverage rate is repeatedly printed, the residual toner is insufficiently cleaned, and sometimes an image fault is generated. The image fault is significantly generated particularly when the image having the high coverage rate is printed.
Conventionally, in order to solve the problem, it is well known that the toner is forcedly consumed to supply the toner to the cleaning device when the low-coverage-rate image is printed. The toner can be supplied to the cleaning device by forming an image patch in a non-print domain, such as an inter-sheet portion.
For example, Document 1 discloses that, in an image forming apparatus, the toner in a development device is recovered by the cleaning device through a photosensitive drum in each 2000 sheets of the images. The operation is performed to prevent a problem caused by the degradation of the toner in the development device. In the image forming apparatus disclosed in Document 1, a developer discharge mode is performed when an average image rate is equal to or lower than a given value.
In view of the problem, Document 2 discloses that, in an image forming apparatus including the cleaning blade, the image patch is formed on the image bearing body during an idling state of the image bearing body and an application of a primary transfer voltage is stopped.
Document 3 discloses that, in an image forming apparatus, density of the specific image patch formed on the photosensitive drum is measured with a density sensor to control the density of the image formed on the sheet. In the image forming apparatus disclosed in Document 3, an intermediate transfer belt is separated from the photosensitive drum during the formation of the image patch, which allows the density of the image patch to be properly measured on the photosensitive drum.    [Document 1] Japanese Patent Laying-Open No. 2011-027931    [Document 2] Japanese Patent Laying-Open No. 2004-093770    [Document 3] Japanese Patent Laying-Open No. 2001-005237
However, in the conventional image forming apparatuses, the toner is consumed more than necessary, the number of images that can be printed for a certain unit amount of toner (for example, for one toner cartridge) is lower than the originally-expected number of images.
For example, when the developer discharge mode is performed in the image forming apparatus disclosed in Document 1, the image patch having a given size is formed in each 2000 sheets. In this case, when the image having the relatively high coverage rate is formed in the latest image formation even if the average image rate (coverage rate) is lower than a given value, the originally unnecessary patch is formed although a relatively sufficient amount of toner is supplied to the cleaning device, and therefore the toner is uselessly consumed. On the other hand, because the patch is formed at the low frequency of every 2000 sheets even if the images having the considerably low coverage rates are successively formed, a lack of toner supplied to the cleaning device is generated, and possibly the image is improperly formed.
In the image forming apparatus disclosed in Document 2, because the image patch is substantially always formed during the idling state of the image bearing body, the consumption amount of the toner that is not used to form the image is increased, and the above problem cannot be solved.
Document 3 does not disclose at all the effective measure to solve the above problem.
The present invention is devised to solve the problem, and an object of the present invention is to provide an image forming apparatus, an image forming apparatus controlling method, and a control program for the image forming apparatus, in which the toner consumption amount is decreased and the chatter or peeling of the cleaning blade can be prevented to maintain the state in which the image is properly formed.